This invention relates to a communications cable, and more particularly to a fire resistant communications cable.
Communications cables, such as cables used in telephone lines, typically consist of insulated copper cores, the layer surrounding the copper being formed from an insulating polymeric material. The insulated cores may be arranged in the form of twisted pairs or quads and a plurality of twisted pairs or quads may be bundled together and encased within an outer polymeric layer. A screening layer can be interposed between the bundled cores and the outer layer to serve as an earth.
One problem facing the manufacturers of cables is that the polymeric materials from which cables are formed represent a possible means by which fires can be transmitted through a building because commonly used polymers such as polyolefins (e.g. polyethylene or polypropylene) can be highly flammable in a fire situation. It is therefore known to make cables from a fire resistant material.
One test used to determine the fire resistance of cables is the so called Steiner Tunnel test (American National Standards Institute ANSI UL 910). The purpose of this test is to determine the flame-propagation distance and optical smoke density for electrical cables that are to be installed in ducts, plenums and other communications spaces and channels within buildings. This test is effectively mandatory in the USA for cables which are to be installed in buildings.
The test involves mounting the cable in a specially designed tunnel or chamber and subjecting the cable to a test fire fuelled by methane gas. An array of thermocouples is used to monitor the propagation of the flame along the cable and a photoelectric cell is used to monitor the density of the smoke created by the resulting fire. In order to meet the requirements of the test, the following criteria must be satisfied:
(a) The maximum flame propagation distance must not be greater than 5 feet (152 cm) beyond the initial test flame. PA1 (b) The peak optical density of the smoke produced is to be 0.50 or less (32% light transmission). PA1 (c) The average optical density of the smoke produced is to be 0.15 or less. PA1 a plurality of cores through which communications signals can be transmitted, each core comprising a metallic conductor surrounded by a close-fitting sleeve of insulating material which is substantially free of halogenated polymers, the insulating material having a permittivity of no greater than 3, and being constituted by or containing a layer of foamed polymer, and wherein at least in the region of the insulating material immediately adjacent the metallic conductor, the polymer contains no fire retardant metal hydroxide and/or carbonate filler; PA1 a first fire protection layer disposed radially outwardly of and surrounding the plurality of cores, the first fire protection layer comprising a fabric formed from inorganic fibres; PA1 a second fire protection layer disposed radially outwardly of and surrounding the first fire protection layer, the second fire protection layer being formed from an extrudable polymer containing a fire retardant metal hydroxide and/or carbonate filler, the first and second fire protection layers not being adhesively bonded together; and PA1 a metallic or metallised screening layer disposed between the cores and the second fire protection layer.
Polymeric insulating materials typically used for covering copper cores in electrical and communications cables include polyolefins such as polyethylene and polypropylene, which are highly flammable in fire situations. In order to overcome this problem, it has been proposed to use as the insulating polymer, a polymer composition which has better fire resistance or fire retardant properties. This approach is exemplified by DE-C-3044871 which discloses a cable in which individual metal conductors are covered with a layer of a fire retardant filled polyvinylchloride.
EP-B-0107796 discloses an optical communications cable in which the optical fibre is encased in a sheath or layer of a fire retardant polyolefin copolymer such as EVA filled with a metal hydroxide, an outer sheath of a similar fire retardant polymer also being provided.
EP-B-0526081 discloses electric and communications cables in which a tape of flexible mineral material is wrapped around the core, the tape being adhesively bonded to an outer layer of a fire retardant filled polymer which forms a char when exposed to a fire situation. The purpose of bonding the tape to the outer layer is to ensure that the char remains as a cohesive protective layer and does not fall away from the cable.
EP-0268827 discloses a fire-resistant electrical cable comprising a conductor surrounded by an insulating layer which in turn is surrounded by a tape-wrap layer which can be formed from metal, woven glass fibre, polyimide, polyimidine, or aromatic polyamide tape having an adhesive on its inner surface.
DE-A-3833597 discloses a fire resistant cable comprising a conductor surrounded by a thin layer of high temperature resistant polymer such as a polyesterimide, a polyetherimide, a polyamidimide or a polyimide, and a thicker outer layer of a non-high temperature stable polymer which is filled with a substance such a aluminium hydroxide.
WO-A-96/25748 discloses a fire resistant cable construction in which the conductor is surrounded by an inner layer of a foamed polymeric material such as polyolefin, a polyolefin copolymer or a polyurethane which preferably contains a fire retarding agent such as magnesium hydroxide. The inner layer in turn is surrounded by a halogenated polymeric layer which also contains a fire retardant additive such as magnesium hydroxide.
U.S. Pat. No. 4,810,835 discloses a coaxial cable in which the conductor is surrounded sequentially by concentric layers of an insulating material, a screening layer, a metallised fibre glass cloth layer and an outer layer of an insulating material.
GB-A-2128394 discloses an electrical cable in which the metal conductor is surrounded by a polymeric insulating material which is filled with inorganic fire retardant agents such as aluminium trihydrate and antimony pentoxide.
Of fundamental importance to the acceptability of communications cables are the electrical properties of the cable, and the typical properties that communications cables should possess are summarised in WO-A-96/25748. One important property is the dielectric constant or permittivity of the insulating material surrounding the conductor wire, which is a measure of the insulating capability of the material. In general, the higher the permittivity of the insulating material, the thicker the insulating material needs to be in order to provide the required characteristic impedance.
The permittivity of polyethylene is approximately 2.3 which makes it an excellent insulating material but, as pointed out above, polyethylene is flammable. Replacing polyethylene with polymer compositions containing fire retarding agents, as disclosed in the documents referred to above, whilst potentially offering improved fire resistance, would be detrimental to the electrical properties and in particular would lead to increased permittivity and therefore the required size of the core.
In order to provide improved fire resistance properties without sacrificing the electrical properties of the insulating material, fluorinated ethylene-propylene polymers (FEP) have been used as the insulation materials for metal conductors. Bundled FEP cores encased within an outer cable sheath formed from a filled fire resistant polymer are understood to have passed the Steiner Tunnel Test; indeed, it is understood by the present applicants that cables of such construction are the only communications cables to have passed the test prior to the present invention being made. However, a major problem with FEP, as stated in WO-A-96/25748, is that it is expensive and often in short supply. Moreover, it is understood that the thermal breakdown products of such fluorinated polymers are toxic.
It is clearly undesirable from a manufacturer's view for the basic raw materials for its products to be difficult and expensive to obtain. It is also undesirable to use a material where the breakdown products of the polymer are toxic fluorine-containing gases. An object of the present invention therefore is to provide a cable in which the need to use fluorinated polymers for the insulation of cable cores is avoided.